how to Airplay music from iMac connected to internet via ethernet to remote speakers connected to Airport Express

The best way to answer is giving you screenshots.

I got a Gen5 AE.. set it to factory default .. and it is seen in the airport utility.

Select the Extreme

It starts the setup wizard.

I have it setup plugged into main router which it recognises but is non-apple so starts with create a network. I type in the network name for wifi and the Extreme name.. identical is fine.. and password.

That is all there is to it.. wait for the setup to complete. Done

The configured AE appears in the Airport Utility. click the icon. Click edit.

Base Station is first tab.. you can reset the name and password if it does not conform to the rules I gave. You should do it via full reset but you can do it this way as well.

Internet in my case is plugged into existing router. No changes are needed at this stage although setting static IP can help in some cases.

The Internet Options has important change.. set IPv6 to link-local only.

Wireless network is fine as per setup wizard. No need to change anything in wireless options at this stage.

Network in my case is Off (bridge mode).. if you are using the Extreme as the main router this should be configured as DHCP and NAT.

That is it.. Extreme is working.

I will post express setup in a new post as limit to number of screenshots I can include.

airport express rollback firmware to 7.6.1 - Apple Community

https://www.macobserver.com/tips/quick-tip/macos-roll-back-airport-basestation-firmware/

more

https://www.google.com/search?q=downgrade+airport+express+firmware+7.6.1&oq=airport+express+downgrade&aqs=chrome.2.69i57j0l4.5403j0j7&sourceid=chrome&ie=UTF-8

Reset the Express to factory.

It is picked up by the Airport Utility .. Click the Other Wi-Fi Device (1)

Select the Express

This will start the wizard and it will by default attempt to extend another wireless airport router.

Wait to this completes.. should just be a minute or two.

Now it shows in the airport utility.. dotted line means wireless connection.

Click the icon of the express. Note the firmware is 7.6.9 .. it works much better than 7.8.1 which is problematic. Click edit.

Base Station should not need any changes. But if you are manually setting up do the fix to the setup here.

Internet is fine.. again we will go back if there are issues and set a static IP..

Change the IPv6 to Link-local only in the Options tab.

Wireless is set by default to extend. If you plugged it via ethernet it will show create a wireless network.

The other option is to join a wireless network.. this allows the Express to work as a wireless client without extending the wireless. Use whatever works for your setup.. with two express it might be better to extend with the one closest to the Extreme and join with the other.

Nothing else should need changing.

I chose the "Router Mode" as "Off (bridge mode)" even though I thought my AE is my main router. I guess I'm confused on what that means. (A few days ago, I asked my Fios ISP to turn off the WiFi that their issued router/modem produces by default. I was hoping to reduce network congestion by eliminating a signal that I was not using. When I look at their device, however, there is a Wifi symbol illuminated on the front of the device, so I think it is actively producing a WiFi signal now...)

Whether the FIOS router has wifi off or on it is still your main router.. and the AE should be in bridge mode..

Unfortunately Apple made the terminology complicated.. the router mode OFF with the qualifier (bridge mode) does not make it any easier to understand. The big value of using the automatic setup is the detection of the setup is right 90% of the time.

To find out if the FIOS wireless is still on is easy..

Hold down the option key and click the wireless fan.

This will give you the list of wireless AP names in your area.. it could be very long if you live in apartment building.. or very short as mine is here in the ManCave.

You can also open wireless diagnostics..

Click on Open Wireless Diagnostics.

A screen will popup.. Do NOT click Continue.. Do NOT run the wireless diagnostics.. it is completely useless. Leave it in the background. In the top menu click on window and scan.

Once scan is open you can close the first popup menu.

Scan will give you all the details on the wireless around you that the computer you are using can detect. It shows channels, RSSI which is signal level etc. You will quickly see if the FIOS is working or not because it usually includes FIOS in the wireless name.

Downgrading firmware is easy.. hold down Option key.. and click the firmware version.

A list will appear.

Pick the one you want.

Not sure what happened.. but you do realise an A1301 Extreme is about 10-12 years old.. and desperately in need of an upgrade.

What model is the Express?

There is a special bridge in the Airport router that links ethernet to wifi.. that bridge can break down so items on wifi are isolated from ethernet and visa versa.

What I would do is reset the Extreme to factory and redo the configuration.. use all new names, very short with no spaces and pure alphanumerics. Passwords same rules but longer. Please don't disregard this one as it is important.

Once you configure the Extreme.. reset the Express and also reconfigure it. I presume it is using extend wireless which should work automatically depending on which express you are using.

No luck then we need more info.

Open airport utility on the Mac.. Click the icon of the Extreme and Express.. take a screenshot of each showing the summary. Post this here. Do this with wifi on the Mac running and then do the same thing with ethernet.. please post the screenshots of each different setup.. even if the Express is missing in the later test.. it should be possible to force a connection to it.